Method and apparatus for the integrated cleaning of a paper-pulp processing plant

ABSTRACT

A method for the integrated cleaning of a paper-pulp processing plant including one or more high density cleaners comprising the steps of supplying the rejected flow ( 10 ) from one of the high density cleaners to a screening centrifugal separator ( 11 ); screening out contaminants in the separator ( 11 ) using a screen hole size of less than 3.5 mm; thickening any fibrous material accepted by the separator ( 11 ) and returning same to stock being processed in the processing plant; and recovering liquid (B) from any material (A) rejected by the separator ( 11 ) prior to disposing of the material. The invention further comprises an apparatus for performing the method.

BACKGROUND OF THE INVENTION

This application claims priority from European Application 08155077, filed Apr. 24, 2008.

The present invention relates to a method and apparatus for the integrated cleaning of a paper-pulp processing plant used primarily, but not exclusively, for the production of corrugated paper and cardboard. The invention is particularly concerned with the cleaning of a plant used for processing paper-pulp produced from recycled paper and cardboard.

In order to make paper, an aqueous suspension containing cellulosic fibres, and optional fillers and additives, is fed through pumps, screens and cleaners, which subject the stock to high shear forces. The resulting pulp is fed into a headbox which ejects the suspension onto a forming wire. Water is drained from the suspension through the forming wire so that a wet web of paper is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine. Drainage and retention aids are conventionally introduced at different points in the flow of the suspension in order to facilitate drainage and increase adsorption of fine particles such as fine fibres, fillers and additives onto the cellulose fibres so that they retained with fibres on the wire. Examples of conventionally used drainage and retention aids include organic polymers, inorganic materials, and combination thereof.

Recycled paper and cardboard can be used to produce pulp for use in the above process. However, the pulp produced tends to have a higher content of anionic impurities, typically colloidal impurities, than wood pulp. The presence of these anionic colloids has an adverse effect on the paper production process. Being anionic, the impurities are unable to attach themselves to the fibres of the pulp in the suspension, which are also anionically charged, and in time tend to accumulate in the equipment comprising the production cycle. This interferes with the production process in a negative way. Firstly, it decreases the quality of the paper produced, for example by producing holes, dark spots and a reduction in the mechanical resistance of the paper. Secondly, it neutralizes any cationic additives used within the process so that they are rendered less effective, which requires greater quantities of these additives to be added than would otherwise be the case. Thirdly, the accumulation of the anionic colloids in the process machinery also results in more frequent breakdowns of this machinery, for example caused by obstructive sediment that also tends to cause the accumulation of dirt. It also generally impedes the flow of the pulp through the machinery itself. Ultimately, all of these factors have an adverse effect on the quality of the resulting paper. Overall, therefore, the efficiency of the process is reduced which leads to increased costs.

In a paper-pulp processing plant, the aqueous suspension containing cellulosic fibres, which is hereinafter referred to as the ‘stock’ is subjected to treatment in a plurality of different baths. Typically, the rejected flows egressing from these baths are also treated to recover fibres that can be re-introduced into the stock order to reduce undue wastage of material. However, when the cellulosic fibres have been produced from recycled paper and cardboard this often causes the unwanted anionic colloidal material to be reintroduced into the stock. In addition, the recovered liquid is also recycled back into the plant at the beginning of the pulping process. It would, therefore, be advantageous for a cleaning arrangement to be incorporated into a conventional processing plant whereby these anionic colloids and other similar colloidal material can be removed from the plant as part of an integrated process.

The object of the present invention is to provide such a method and apparatus for the integrated cleaning of a paper-pulp processing plant in order to remove or substantially reduce the quantity of pollutants, specifically anionic colloids, from the plant. The term ‘integrated cleaning’ means that the cleaning process is integrated into the pulp processing process and operates continuously as the pulp processing is in the operation rather than being a separate cleaning process that is carried out during downtime of the plant.

It is an additional object of the present invention to reduce the liquid content of any unwanted material in order that it can be readily disposed of, for example, by dumping or incineration.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a method for the integrated cleaning of a paper-pulp processing plant including one or more high density cleaners having the steps of:

supplying rejected flow from a high density cleaner, and preferably a final high density cleaner, to a screening centrifugal separator;

screening out contaminants in the separator using a screen holes size of less than 3.5 mm;

thickening any fibrous material accepted by the separator and returning same to stock being processed in the processing plant; and

recovering liquid from any material rejected by the separator prior to disposing of the material.

The invention also includes a paper-pulp processing plant including one or more high density cleaners and incorporating apparatus for the integrated cleaning of same in accordance with the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process diagram showing apparatus forming part of a paper-pulp processing plant for use in accordance with the present invention for the removal of pollutants from the stock; and

FIG. 2 is a vertical cross section of a compactor press comprising one of the pieces of apparatus forming part of the process shown in FIG. 1 for the removal of pollutants from the stock.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention rejected flow from the final high density cleaner in a conventional pulp processing plant is subjected to a cleaning process prior to its reintroduction into stock provided at the commencement of the process. A high density cleaner is a feed stock (pulp) cleaner that removes undesired high density contaminants from the pulp. The cleaning process is, therefore, integrated into the pulp processing process as a whole.

Preferably, the contaminants in the separator are screened using a screen hole size of approximately 2.5 mm. This ensures that colloidal pollutants are screened out of the liquid which is recovered from the separator prior to its return to the stock.

According to a second aspect of the present invention there is provided a paper-pulp processing plant including one or more high density cleaners and incorporating apparatus for the integrated cleaning of same, the apparatus comprising:

a screening centrifugal separator set up to receive the rejected flow from one of the high density cleaners and adapted to screen out contaminants from the rejected flow using a screen hole size of less than 3.5 mm;

a thickener for thickening any fibrous material accepted by the separator and returning same to stock being processed in the processing plant; and

apparatus for recovering liquid from any material rejected by the separator prior to disposal of the material.

Hence, in the present invention the rejected flow from the final high density cleaner in a conventional pulp processing plant is subjected to a cleaning process prior to its reintroduction into the stock at the commencement of the process. The cleaning process is, therefore, integrated into the pulp processing process as a whole.

Further preferred but non-essential features of the various aspects of the present invention are further described in the dependent claims appended hereto.

The various aspects of the present invention will now be described by way of example with reference to the accompanying drawings.

In the present invention, the rejected flow from the final high density cleaner in a conventional pulp processing plant is subjected to cleaning process as will now be described with reference to FIG. 1.

The rejected flow 10 is supplied to a screening centrifugal separator 11 that preferably comprises a conventional ‘Separplast’ separator available from numerous manufacturers that has been modified as described below. Separplast separators were designed to separate light contaminants such as plastics, polystyrene, hot melts and the like originating from recycled paper waste from stock. Conventionally, in a paper-pulp processing plant a Separplast separator is positioned after the pulper and the high density cleaner. The accepted flow is then recycled back to the pulper and the rejected flow is thickened for disposal. They are not, however, designed to remove colloidal waste.

Centrifugal separators such as the Separplast machine operate by rotating a cylinder carrying rotor blades at high peripheral speed inside a perforated basket. The rotor typically has an overall diameter of 997 mm, although various sizes of machines could obviously be made. The rejected flow enters the separator 11 at the top from a constant level box 12 which regulates the flow and is provided with an on/off valve (not shown). Within the separator 11 the flow passes down through the rotor blades and the basket where it is deflaked and screened. Lateral sprinklers spray clarified washing water onto the basket to lubricate the zone between the basket and the rotors and to prevent the clogging. ‘Accepted’ fibrous material passes through the basket laterally along flowpath 13 while rejects are caught and, after being washed by flushing water that minimizes their fibre content, are conveyed to a strainer to separate them from the water. The rejects exit from the lower part of the separator along the flowpath 14.

In a conventional Separplast separator 11, the basket has a screen hole diameter between 3.5 and 6 mm dependent on the degree of cleaning required. However, in the present invention, the screen hole diameter of the basket is reduced to less than 3.5 mm and preferably to around 2.5 mm in order to separate out the colloidal pollutants that it is desired to catch. Reduction of the screen holed diameter can reduce flow through the basket and, therefore, regulation of the flow by the constant level box 12 and its consistency is important. Sufficient water must be provided to lubricate the zone between the basket and the rotors. This is achieved by increasing the pressure of the clarified water sprayed onto the basket from a pressure of around 1.5 bar in a conventional Separplast machine to a pressure around 4 bar. The nozzles of the sprinklers spraying this water are also increased in size from a diameter of around 1 mm to around 2 mm. In addition, the diameter of the rotor is increased for a tighter fit in the basket, e.g. from around 997 mm to around 999 mm. All of theses changes have the effect of improving the output of the separator 11 by reducing clogging of the basket. In addition, the pressure of the flushing water used to wash the rejects is increased from around 2 bar in a conventional separator 11 to around 6 bar in the separator 11 of the present invention. This increased quantity of flushing water reduces to a minimum the presence of usable fibres in the rejects. All of the washing and flushing water is recovered and reintroduced into the initial pulping bath so that any usable fibres mixed with the water are reintroduced into the process and are not lost.

The overall effect of these modifications is that there is a significant increase in the quantity of water used, e.g. from around 250 litres per minute to around 600 litres per minute. Hence, whereas a conventional Separplast machine operates at an inlet consistency ranging between 3.5% and 5% fibre density with an accept consistency slightly lower, by around 0.2%, than inlet consistency owing to the addiction of washing water, in the modified Separplast machine of the present invention the accept consistency is of the order of 0.1% to 1.5% and the reject consistency is of the order of 0.5%.

The accepted flow passing along flowpath 13 can be reintroduced into the stock being processed in the main processing apparatus, preferably at an initial part of the cycle. However, before reintroduction it is preferably thickened to prevent undue dilution of the stock. The thickening is preferably carried out using an inclined screw thickener 15 which receives the accepted flow from Separplast machine 11. Such a thickener 15 has a slow mechanical movement which drains the liquid from the fibres using gravity so that the fibres are effectively ‘squeezed’ together. There is, therefore, a reduction in volume of the fibrous material passing through the machine as it rises through the machine from its lower inlet to an upper outlet 16. Liquid draining from the stock is filtered prior to egressing from the thickener 15 via a bottom outlet 17. Overall, the increase in fibre density of the accepted flow is from around 1.5% to between 4% and 5% prior to reintroduction into the stock via a screening bath 18. The liquid egressing from the outlet 17 is also reintroduced into the process at the initial pulping stage.

Preferably, the screw thickener 15 is also modified to increase the quantity of fine usable fibres in the accepted flow that would otherwise be lost in the liquid draining from the fibrous material rising through the thickener 15. These modifications involve altering the filters which are used to filter the liquid egressing from the outlet 17 so that the size of the filter holes is reduced from a diameter of 2.0 mm to a diameter of 1.25 mm. In addition, the speed of rotation of the screw within the thickener 15 is reduced by approximately 20% over the speed in a conventional screw thickener of this type. This reduces the fibre content of the filtered liquid egressing from the outlet 17 from approximately 600 parts per million to around 200 parts per million.

The rejects in the flowpath 14 are also treated to separate them from the liquid with which they are mixed on egress from Separplast 11. Disposal of the solid or colloidal rejects is typically by incineration or dumping and in both cases it is necessary to reduce the liquid content of the rejects so that they are around 70% dry. To this end the material in the flowpath 14 is firstly fed to a second and inclined screw thickener 19. As the feed to the screw thickener 19 may be intermittent, a feeding tank (not shown) may be interposed in the flowpath 14 to feed the thickener 19 to dampen or even out the flow therto and to supply material to the thickener 19 with a fibre density of around 2%.

The screw thickener 19 is adapted differently from the thickener 15 as it has to deal with larger solid waste and with the colloidal reject material. In this case, therefore, the filters which are used to filter the liquid egressing from the outlet are increased in size from a diameter of 2.0 mm to a diameter of 2.5 mm. In addition, the speed of rotation of the screw within the thickener 15 is increased by approximately 15% over the speed of a conventional screw thickener of this type. These modifications prevent the solid waste from clogging the water outlet from the thickener 19 while still allowing a thickening of the reject flow to density of around 12%. The liquid egressing from this thickener 19 is again reintroduced into the process at the initial pulping stage but it will be appreciated that the thickened output is still too watery to be dumped or incinerated. The output from the thickener 19 is therefore fed to a compactor press 20 that increases the density of the reject flow to almost 70%. This thickened material is then finally discarded as scrap via a bin 21 which catches the solid waste from the press 20. The liquid separated therefrom can then also be reused within the process after suitable clarification treatment.

The compactor press 20 is provided with a large inlet hopper 22 (shown in FIG. 1) that is located directly beneath the outlet from the screw thickener 19. This is important because the hopper 22 must store the output from the thickener 19 whilst material already within the press 20 is compacted, the press 20 operating in a batch mode whereas the thickener 19 acts continuously. Within the press 20, as shown in FIG. 2, material, marked A, from the hopper 22 falls into compression chamber 23 wherein it is acted on by a hydraulic ram 24 and pushed upwards into an upwardly curving channel 25 at one side of the press 20. This action causes a first compaction of the material and the resulting released liquid, marked B, drains out press 20 through a bottom, self-cleaning grill 26 and into a sump (not shown). This liquid can then be screened and recycled back into the main pulp processing process, as indicated above. The inclination of the channel 25 upwards also encourages liquid held within the material in the channel 25 to drain out through the grill 26.

The ram 24 reciprocates within the chamber 23 with a pressure of around 21 kg/cm² so that on each stroke, more material A can be admitted to the chamber 23 from the hopper 22. The material which is pushed upwards into the channel 25 by the ram 24 is eventually pushed upwards against a spring-loaded, top-hinged door 27 located at the top of the channel 25 as further material is pushed into the bottom of the channel 25 by the ram 24. The spring loading 28 applied to the door 27 is adjustable so that the degree of compaction achieved within the press 20 can be varied as required and is typically equal to a constant force, e.g. around 44,000 kg. Ultimately, the upward pressure of material within the channel 25 overcomes the spring loading 28 and the material. marked C, is ejected through the door 27 and out of the press 20 so that it can fall into the bin 21. The contents of the bin 21 can then be directly disposed of by either dumping or incineration.

The compacted material in the bin 21 comprises plastic, pieces of wood, and the colloidal pollutants that would otherwise cause problems within the equipment of the pulp processing plant as described above. The integrated cleaning arrangement of the present invention enables these pollutants to be efficiently and cost effectively removed from the process with the following consequent advantages:—

-   -   a significant reduction in the impurities present in the paper;     -   an improvement in the productivity of the processing plant by         reducing the number of breakdowns caused by accumulation of the         pollutants;     -   a significant reduction in downtime of the plant required for         routine cleaning and washings;

a reduction in the quantity of chemical additives required within the pulp processing;

-   -   an increase in of the mechanical characteristics of the         resulting paper without altering the stock or percentage of         starch used within the process;     -   the possibility of using the plant to make paper with superior         characteristics and having a greater value whilst still using         recycled paper; and     -   a reduction in the quantities of waste paper and chemical         additives required to produce the same quantities as previously,         i.e. a more efficient use of the raw materials. 

1. A method for the integrated cleaning of a paper-pulp processing plant including one or more high density cleaners for removing high density impurities from feed pulp, comprising the steps of: supplying rejected flow (10) from at least one of the high density cleaners to a screening centrifugal separator (11); screening out contaminants in the separator (11) using a screen hole size of less than 3.5 mm; thickening any fibrous material accepted by the separator (11) and returning same to stock being processed in the processing plant; and recovering liquid (B) from any material (A) rejected by the separator (11) prior to disposing of the material.
 2. A method as claimed in claim 1, wherein the rejected flow is from a final high density cleaner and contaminants in the separator (11) are screened using a screen hole size of approximately 2.5 mm.
 3. A method as claimed in claim 2, wherein clarified water is sprayed onto the contents of the separator (11) at a pressure in excess of 1.5 bar via nozzles having an aperture in excess of 1 mm in diameter.
 4. A method that is claimed in claim 2, wherein the material (A) rejected by the separator (11) is flushed with water at a pressure in excess of 2 bar prior to its egress from the separator (11).
 5. A method as claimed in claim 1 comprising the additional step of supplying the rejected flow (10) to the separator (11) via a constant level box (12).
 6. A method as claimed in claim 1 comprising the additional step of thickening any fibrous material accepted by the separator (11) prior to returning same to the stock and thickening any material (A) rejected by the separator (11) prior to its disposal.
 7. A method as claimed in claim 6, comprising the additional step of compacting the thickened material rejected by the separator (11) in order to recover the liquid (B) from the thickened material (A) prior to disposal of the thickened and compacted material (C).
 8. A paper-pulp processing plant including one or more high density cleaners and incorporating apparatus for the integrated cleaning of same, the apparatus comprising: a screening centrifugal separator (11) set up to receive the rejected flow (10) from one of the high density cleaners and adapted to screen out contaminants from the rejected flow using a screen hole size of less than 3.5 mm; a thickener (15) for the thickening any fibrous material accepted by the separator (11) and returning same to stock being processed in the processing plant; and apparatus for recovering liquid (B) from any material (A) rejected by the separator (11) prior to disposal of the material.
 9. A plant as claimed in claim 8, wherein the screening centrifugal separator comprises a screening basket with a screen hole size of approximately 2.5 mm.
 10. A plant as claimed in claim 8 comprising a constant level box (12) to supply the rejected flow (10) to the separator (11).
 11. A plant as claimed in claim 8 comprising a first inclined screw thickener (15) adapted to thicken any fibrous material accepted by the separator (11) prior to returning same to the stock, the screw thickener (15) comprising liquid filtering holes with a diameter less than 2.00 mm.
 12. A plant as claimed in claim 8 wherein the means for recovering liquid (B) from the material (A) rejected by the separator (11) comprises a second inclined screw thickener (19) adapted to thicken any material rejected by the separator (11) and comprising liquid filtering holes with a diameter greater than 2.00 mm.
 13. A plant as claimed in claim 8 wherein the apparatus for recovering liquid (B) from the material (A) rejected by the separator (11) also comprises a compactor press (20) adapted to compact the output from the second inclined screw thickener (19) and to release the liquid (B) therefrom.
 14. A plant as claimed in claim 13, wherein the compactor press (20) comprises a compression chamber (23) in which a ram (24) reciprocates to push material upwards into an inclined channel (25) against the force of a spring-loaded door (27) the spring loading (28) applied to the door (27) being adjustable so that the degree of compaction achieved within the press (20) can be varied as required.
 15. A plant as claimed in claim 14, wherein the spring loading (28) applied to the door (27) is equal to a constant force of around 44,000 kg. 